There are many instances in which it is desirable to generate electrical energy at a fixed frequency using a generating machine that is coupled to an input shaft driven by a variable speed source. For example, utilities in many parts of the world use modern wind machines to augment the supply of the electrical energy they provide to their customers from more conventional power plants.
Generally, the electrical energy output from these wind machines is connected to the utility's power grid, which preferably is maintained at a fixed frequency, such as 50 or 60 Hz. In order to couple the wind machines to the power grid, it is necessary that the electrical energy produced by the wind machines be substantially synchronized in phase and frequency with the power grid.
A conventional induction machine can be employed as a generator to produce electrical energy at a frequency that is a function of the speed of the machine's rotor and the number of poles in the machine. However, conventional induction machines have many limitations when used as generators on variable speed input power sources, such as wind machines. Consequently, it has been necessary to develop various measures to enable variable-input-speed generating machines to generate electrical power at a fixed frequency matching that of the power grid. One solution to the problem is to use brushless doubly-fed induction machines, which, when properly controlled and connected to a power grid in an appropriate manner, can generate electrical energy at a frequency matching that of the power grid, independent of the rotational speed of the machine's input shaft, at least within a limited range.
Examples of doubly-fed induction machines are disclose in U.S. Pat. Nos. 3,183,431; 3,571,693; 4,229,689; 4,246,531; 4,305,001; 4,472,673; and 4,701,691. All of these patents disclose brushless doubly-fed induction machines having two stators and two rotors, so that when one of the stator windings is energized with an excitation current, the other produces an output current to drive a load. In all of these prior art doubly-fed induction machines, the dual rotor windings are interconnected, usually in reverse phase sequence.
FIG. 1 shows a brushless doubly-fed induction machine 100, of the type disclosed in the above-referenced prior art patents. This machine is typically used as a variable speed constant frequency AC generator, but is also usable as a motor. Brushless doubly-fed induction machine 100 includes a cylindrical shell 102 that serves as its housing and is coupled at opposing ends to a pair of end caps 103 and 104, in which bearings 116 and 126 are respectively mounted. Substantially-identical laminated stator structures, including a first stator 105 and a second stator 118, are mounted end-to-end within cylindrical shell 102 around a shaft 114. Laminated rotor structures including a first rotor 110 and a second rotor 124 are mounted on shaft 114, and opposite ends of the shaft are supported by bearings 116 and 126, respectively. Both the first and second rotors are wire wound. Shaft 114 is driven at a variable speed by an external source (not shown).
The rotation of shaft 114 by the variable speed external source also rotates both first rotor 110 and second rotor 124. The first rotor is inductively coupled to magnetic flux produced by the excitation current flowing in first stator 105, but is substantially magnetically isolated from second rotor 124 and second stator 118. Second rotor 124 is inductively coupled with second stator 118, but substantially magnetically isolated from the first stator. First and second stators 105 and 118 are respectively wound with polyphase distributed windings 106 and 120. First stator windings 106 are connected to an external polyphase frequency excitation source 138, which provides an excitation current, while second stator windings 120 are connected to an output load 140, typically an AC mains. First and second stator windings 106, 120 may comprise the same, or different numbers of poles and phases, dependent upon one or more of: (a) the characteristics of polyphase frequency source 138; (b) the input speed range of shaft 114; and, (c) the load.
First rotor 110 and second rotor 124 respectively include rotor windings 108 and 122, comprising polyphase distributed windings of the type commonly used in wound rotor induction machines. Each rotor has the same number of poles as the stator with which it is inductively coupled, and both rotor windings have the same number of phases. Rotor windings 108 and 122 are connected together by plurality of conductors 136, forming a closed electrical circuit, either in an in-phase sequence, or in a reverse phase sequence (in which the flux in second rotor 124 rotates about shaft 114 in a direction opposite that of the flux in first rotor 110).
Brushless doubly-fed induction machine 100 operates in the following manner when connected to a passive load. As shaft 114 is rotated at a given speed, slip frequency currents are generated in first rotor 110, as a result of first rotor 110 rotating within the magnetic flux produced by the excitation current flowing in first stator 105. Second rotor 124 is reverse phase connected to first rotor 110 to receive the current induced in the first rotor. If both the first and second rotors are wound with the same number of poles and first stator 105 is excited with DC current, the resulting current flowing in second rotor 124 produces a magnetic field that is inductively coupled to second stator 118 and which rotates in space at twice the shaft rotation rate. Exciting stator 105 with AC current rotating in the same direction as the shaft subtracts the input frequency from the frequency of the rotating magnetic field applied to stator 118, hence reducing the output frequency of stator 118. Exciting stator 105 with AC current rotating in the opposite direction as the shaft adds the input frequency to the stator 118 output frequency. Thus, the output frequency is the algebraic sum of the input frequency and the shaft rotation frequency, allowing the output frequency to be controlled even through the shaft speed varies.
Although all of the foregoing patents teach brushless doubly-fed induction machines and control systems that employ wound rotors, none of the patents discloses a brushless doubly-fed induction machine that employs dual cage rotors. In general, induction machines that employ cage rotors, which are commonly called "squirrel-cage" rotors due to their similarity in appearance to the cylindrical cages in which squirrels exercise, are preferred over induction machines that employ wound rotors. Cage rotors are usually less expensive to manufacture (for the same output rating), and are very rugged. In view of the need to provide lowcost generators for alternative energy source applications such as wind machines, it will be apparent that there is a need for providing a generator that can be driven at a variable speed with the cost saving advantages of cage rotors. Accordingly, it would be advantageous to provide a brushless doubly-fed induction machine that employs dual cage rotors instead of wound rotors.